1. Field of the Invention
The invention relates to a one-component, reactive, moisture-crosslinking composition based on polydimethylsiloxane-urea/urethane copolymers with alkoxysilane end groups and to the use thereof as a hotmelt adhesive.
2. Background Art
Hotmelt adhesives, or simply hotmelts, are generally physically setting adhesives which at room temperature are present as one component in solid, substantially solvent-free form.
The polymer components or binders of such hotmelt adhesives have a predominantly linear catenary structure and at room temperature are in an amorphous, glasslike or partially crystalline state. In order to attain and set further specific properties such as cohesive strength, viscosity, softening point or setting rate, for example, further additions may be necessary in the adhesive formulation. These additions include tackifying resins for improving the wetting properties and raising the adhesion, plasticizers for raising the flexibility and lowering the melt viscosity, stabilizers and antioxidants for preventing oxidative alteration during processing of the melt under the influence of oxygen, and also for improving the aging behavior of the bonded joint. Additionally, fillers may be used to increase strength and possibly to reduce costs. The principal hotmelt adhesives are based on binder systems such as polyurethanes, epoxy resins, polyamides, ethylene-vinyl acetate copolymers, styrene block copolymers, saturated polyesters, polyolefin copolymers, synthetic rubbers, and mixtures of these systems.
Whereas the amorphous polymers soften over a more or less broad temperature range, the crystalline or partially crystalline polymers display a more or less sharply defined melting point. Amorphous systems of copolyesters, for example, often still exhibit, even at high molar masses, a cold flow and are therefore of only limited usefulness or can be used only in combination with very high molecular mass copolymers.
One way to increase the cohesive strength and dimensional stability under thermal load (adhesive properties even at a relatively high service temperature) of hot melt adhesives is to use reactive adhesives. Reactive formulations of this kind, as a particular form of hotmelts, are known that constitute a combination of physically setting and chemically reacting systems. For this purpose, hydroxy-functional polyesters are generally reacted with an excess of diisocyanates and isocyanate-terminated polymers are prepared therefrom. These polymers are then able to cure, with crosslinking, on ingress of atmospheric moisture. Since a number of weaknesses are inherent in polyurethane hotmelt systems, such as isocyanate monomer emission (monomeric aromatic diisocyanates such as MDI or TDI, and/or their corresponding amines, are suspected of being carcinogenic), CO2 elimination, which leads to formation of bubbles, or the yellowing tendency in the case of aromatic isocyanates, numerous systems are also being developed at present based on silane-crosslinking polymers, which in terms of the aforementioned disadvantages represent a promising alternative. The prepolymers are in this case reacted with silane-functional monomers, so that from said monomers it is likewise possible to prepare moisture-postcrosslinking hotmelt adhesives.
For greater ease of processing, preference is given to one-component systems, since they are easier to apply and can be made subject to automation. Since in the case of reactive hotmelt adhesives the postcure rate formulated for such one-component compositions is usually moderate, in order to ensure sufficient stability on storage, it is more difficult to vary the profile of properties. Problems associated with non-postcrosslinking systems here, conversely, are generally the dimensional stability under thermal load (cold flow, remeltable), the mechanical properties, and the adhesion to the substrate. With the above-described moisture-curing systems these disadvantages are largely avoided. Two-component compositions usually display a significantly improved profile of properties but present problems in terms of processing. The mixing of the components, using static mixers during application, for example, must be uniform in order to ensure a consistent working time and end quality. It is generally necessary here to make a compromise between cure time and working time. The apparatus required for two-component adhesives is also much more complex and hence the application is usually more expensive.
Organosiloxane copolymers, especially polydiorganosiloxane-urethane and polydiorganosiloxane-urea copolymers, are known. The different systems are described in the overview by I. Yilgör and J. E. McGrath in Adv. Polym. Sci., 1988, 86, pp. 1–86. A multiplicity of further publications and patents deal with specific applications of such block copolymers. Polyurethanes and silicone elastomers are complementary within wide regions. Consequently the combination of the two systems yields materials having innovative, excellent properties. Polyurethanes are distinguished by their mechanical strength, elasticity, and very good adhesion and abrasion resistance. Silicone elastomers, on the other hand, possess excellent temperature, UV stability, and weathering stability and special surface properties (low surface tension). They retain their elastic properties at relatively low temperatures and so also do not tend toward embrittlement.
In the overview by I. Yilgör et al. in Polymer, 1984 (25), pp. 1800–1816 the properties of polydiorganosiloxane-urea copolymers were subjected to closer inspection. The silicone and isocyanate polymer building blocks are readily miscible within a wide range. The mechanical properties are determined by the proportion of the different polymer blocks. The polydiorganosiloxanes form the soft segments and are decisive for the elasticity, while the diisocyanates form the hard segments and are critical to the mechanical properties. The development of hydrogen bonds between urethane or urea binding groups determines the mechanical properties. As a result of the strong interactions of the hydrogen bonds between the urea units, these compositions are generally of very high viscosity or solid at room temperature.
The overviews mentioned above describe and discuss a multiplicity of applications and application possibilities. EP-A-250248 describes the preparation and a possible application of these copolymers for nonstick coatings and pressure-sensitive adhesives. WO 96/34030 describes, furthermore, a possible preparation of polysiloxane-urea copolymers having reactive and nonreactive end groups. Reactive end groups used in that case include alkoxysilanes for the end termination of the polymers.